The island of Hirta, on the western coast of Scotland, is home to a special breed of sheep. Soay sheep, named after a neighbouring island, are the most primitive breed of domestic sheep and have lived on the isles of St Kilda for at least a millennium. They’re generally smaller than the average domesticated sheep, and that difference is getting larger and larger. Over the last 20 years, the Soay sheep have started to shrink.

They are becoming gradually lighter at all ages such that today’s lambs and adults weigh around 3kg less than those from 1986. Their hind legs have also shortened to a similar degree, suggesting that they have indeed shrunk, rather than fallen increasingly ill.

The reasons behind this downward trend have now been revealed by a group of British scientists led by Arpat Ozgul from Imperial College. Using decades’ worth of data, the team showed that natural selection normally favours larger sheep, as the odds of survival increase with body size. But this evolutionary pressure has been overwhelmed by the effects of climate change. Warmer winters have led to easier conditions, and less need to pile on the pounds in the first years of life. The lambs can afford to grow more slowly and they become smaller adults, who are only physically capable of raising small young themselves.

Soay sheep live in a closed population that doesn’t have to deal with human interference, predators, migrants (either in or out), or significant competitors. That makes them an ideal population to study if you’re an evolutionary biologist interested in how animal populations change over time. One such group, including Ozgul and his colleague Tim Coulson, have been studying the Soay sheep since 1985 and have brilliantly called themselves SLAPPED (short for Studies in Longitudinal Analysis of Population Persistence and Evolutionary Demography).

The group wanted to work out the extent to which the sheep’s shrinking size is due to the influence of natural selection and to what extent it is just an ecological response to changing environments. To that end, they developed a mathematical job designed to analyse their 24 years of data and tease apart these contrasting effects.

The model showed that natural selection favours heavier individuals, who are more likely to make it past the first two years of life. But these effects were paltry and largely counteracted by a far more important influence – the difference in body weights between parents and their young. Every August, year on year, the ewes were rearing daughters that were around 150g lighter than they were at the same age.

The sheep are also growing more solely than they used to, putting on about 93g less in their first year than they used to 20 years ago. This explains both why the adult sheep are smaller, and why the birth weights of lambs are falling – ewes simply cannot produce larger young when they themselves are reproducing earlier and failing to reach full adult body size. So today’s Soay sheep run the race to adulthood with a poorer start and a slower pace – no wonder they’re shrinking.

But why are the sheep growing more slowly than they used to? Ozgul says that the answer is climate change. The growth of the Soay sheep turned out to be very sensitive to shifting climate. Since 1980, winters on Soay Island have become warmer, milder and shorter, and grass grows for more of the year. As a result, lambs spend less time depending on their stores of fat for survival, and they have more food to graze on when winter ends.

The result is a more forgiving climate, where even lambs that grow slowly make it past their first year. Compared to earlier generations, these newborns have an easier time of it and they don’t need to put on as much weight in their first few months. Whereas in the past, only the largest and fastest-growing lambs would make it to breeding age, now even the slackers can get away with it. Climate, then, is the culprit behind the mystery of Scotland’s shrinking sheep.

It’s not a question of good or bad. It is a simple question of actual evidence that it is happening. Some species will benefit from global warming and some will not. That’s basic evolutionary biology. The trouble is there are still a large number of people who simply don’t believe it is happening. Of course, there are still people who think the world is flat, big foot is out there and it is only a matter of time before we find that elusive beast in the loch. Of course, we can’t do anything about big foot and we may be too late to do anything at all about the cycle we have created that is causing this period of global warming, but at least we can look at the science and understand what it is telling us.

There’s a nice graph of sheep weight but no graph of temperature.
The Met Office weather station at Tiree – which faces the North Altlantic Drift much as Hirta does – has an average rise in temperature since 1930 of about 0.004 of a degree per year.
Of course, if you already know that it’s man-made climate change that’s shrinking the sheep there’s no need to ask about the temperature. But at least we can look at the science and understand what it’s telling us.http://www.metoffice.gov.uk/climate/uk/stationdata/tireedata.txt

Fascinating.
I can’t help wondering if this is part of the mechanism that develops micro-fauna amongst isolated populations? All of the required factors had been in place for a long time apparently. A relatively small, isolated population, free from predators and direct human influences [even supplemental foods count here]. All that was needed was a nudge across the threshold of speciation [genetic drift at least] in the form of milder climate.
Sounds like what happened to a lot of island populations around the end of the last ice age.

Rich@6
Since the salient point here is winter temperatures and the relevant range is the years 1986 – 2006 (as given by the graph in the article), I ran the METs Tiree numbers for January 1986 – 2006.
January temps from 1986-2006 increase on a linear trend of about 0.058 degC/year or about 1.2 deg C for the 21 year span.
To quote you Rich, But at least we can look at the science and understand what it’s telling us.

Ron, yes, I did have second thoughts. The article was about recent warming so the trend since 1930 is small yet irrelevant. I tried Stornoway too and, for the relevant years got a rise much like yours. I was going to post it but you beat me to it.
We should probably dig deeper to determine the change in the length of the seasons as relevant to the sheep but, really, the point is already made: it got warmer and the sheep got smaller.
I can’t take credit for the quote, though, which is from Donna @4.

The Tiree data I’m looking at (Rich’s link) is monthly data. The number posted above was for the reported monthly max. The monthly minimums show a similar increase for January: .077 degC/year for a trend increase of 1.6 C for the 21 years.
Also, precipitation trend for January is decreasing: -0.94mm/year or -20mm for the 21 years. That’s roughly a 13% drop in January precip.
Warmer temps, less precip.
Can’t speak for the number, length, or severity of cold spells though.